Applicants' invention relates to circuit topologies for accurate logarithmic conversion of the envelope of a modulated signal.
In many types of radio receivers, logarithmic envelope detection is used in various ways for recovering the information transmitted. For example, in receivers for amplitude-modulated (AM) radio signals, log envelope detection can be used for demodulating the received AM signal. After exponential conversion, the original information, i.e., the modulation signal, becomes available. In a typical data receiver, the output of a log convertor provides one of the components needed to decompose the signal into its I and Q components. Furthermore, the d.c. level of the output of a logarithmic envelope detector is an indication of the received signal strength.
Log envelope detectors are known from several patent applications, technical papers, and circuit device catalogues. They are often configured as a cascade of limiting gain sections, each of which consists of a full-wave detector and an amplifier. Summing the output signals generated by the sections produces an accurate logarithm of the input signal.
One of the problems often encountered with these circuits is their sensitivity to parameter dispersion and temperature variations. This leads to rather large errors in the convertor's output voltage.
This sensitivity problem is often solved by measuring the transfer characteristics of each individual circuit and storing the measured transfer characteristics in a memory, such as a ROM, together with required corrections for temperature. Of course, the drawbacks of this strategy are the large effort required for collecting the measurements and the circuit overhead required for adequate error correction, e.g., the temperature must be measured by an additional temperature sensor.
Another problem is the operating range of a logarithmic envelope detector is usually determined by the noise level and the maximum allowable input level. The effective maximum input level is usually increased by using an attenuator to limit the amplitude of the input signal. Of course, the attenuator must not add too much noise or introduce extra temperature dependency. This aim is not always achieved in existing circuits.
One example of a logarithmic envelope detector is the AD640 device made by Analog Devices. The device contains a cascade of limiting gain sections, each consisting of a full-wave detector and an amplifier stage. The maximum input level is determined by an integrated attenuator, which is compensated for the effects of temperature variations, and the noise level is mainly determined by this attenuator.
U.S. Pat. No. 4,990,803 to Gilbert distinguishes between three basic types of logarithmic amplifier (or convertor): (1) using the exponential law of a PN junction; (2) analog-to-digital (A/D) conversion and digital log computation; and (3) successive detection or multi-stage conversion. Examples of the first basic type are described in U.S. Pat. No. 3,569,736 to Tschinkel; U.S. Pat. No. 3,790,819 to Chamran; U.S. Pat. No. 5,200,655 to Feldt; and U.S. Pat No. 5,286,969 to Roberts. Of more interest with respect to Applicants' invention is the third basic type, examples of which are described in the Gilbert patent and U.S. Pat. No. 5,296,761 to Fotowat-Ahmady et al. and European Patent Publication No. EP 0 517 305 to Fotowat-Ahmady et al.
The Gilbert patent describes a log amplifier having a plurality of d.c.-coupled gain stages that are based on differential amplifiers biassed by tail-current generators. The currents supplied by the current generators are proportional to absolute temperature and are compensated automatically for the effects of finite transistor beta and base and emitter resistances. Each stage includes a detector that is implemented by distinct components and that produces the stage's temperature-dependent logarithmic output current. Since each detector loads the forward signal path, emitter followers are included as counter measures.
In Gilbert's log amplifier, the accuracy of the log function increases as the gain decreases, which results in a larger number of sections for a given gain. Also, the gain, which is fixed by the tail currents, must be proportional to absolute temperature and must be compensated for base-current losses. The Gilbert patent stabilizes the gains of its differential-amplifier pairs by controlling the voltages across the collector resistors with an indirect feedback loop that includes a model differential pair. Although potentially more reliable than explicit compensation, all systematic errors (such as transistor beta errors) must be individually and expressly taken into account.
In the log detector described in Fotowat-Ahmady's European publication, like the amplifier described in the Gilbert patent, each stage includes a detector that is implemented by distinct components. On the other hand, the logarithmic output voltage is not obtained from the common emitters of the differential amplifier, but instead an extra differential pair is added at the collectors of the differential gain pair. This introduces more offset problems and influences the small-signal behavior of the gain section because the extra differential pair loads the gain section's output.
The outputs of Fotowat-Ahmady's extra differential pairs are connected to a sense amplifier, which is itself a differential pair. (The other input of the sense amplifier is connected to a dummy circuit.) The sense amplifier includes a resistor for adjusting its transfer gain, and this emitter degeneration resistor contributes to a reduction in the accuracy of the device. Accuracy is further reduced because the device's differential-amplifier tail currents are matched by trimming, which results in unpredictable temperature coefficients.
The log detector described in Fotowat-Ahmady's U.S. patent is different from the detector described in the European publication in a way that tends to improve the proportionality of the detector's response. Also, the bias-current organization in the device described in the U.S. patent is different from that described in the European publication, in which all current sources are intended to have the same temperature coefficient (a condition sought by simulation and trimming). In the device described in the U.S. patent, the bias currents have two different temperature coefficients, and the sense amplifier is biassed by a current that is proportional to absolute temperature. Furthermore, an extra conversion, provided to desensitize the output current, uses a reference tail current.
These features limit the accuracy of the logarithmic conversion obtainable by the device. Moreover, as in the European publication, the transfer characteristics are determined by the emitter degeneration resistors, which indicates the absence of any intent to use the large-signal d.c. properties of the sense amplifier bipolar devices.